1. Field of the Invention
The present invention relates to RF and microwave mixers and more particularly to mixers for up-converting RF and microwave signals which incorporates phase modulation power spreading to reduce the power level of spurious output signals (also known as spurs).
2. Description of the Prior Art
Mixers are generally known in the art and are used in various applications for up-converting or downconverting microwave and RF signals having a frequency f1 to a higher or lower frequency for by way of a local oscillator. Such mixers are non-linear devices with two input ports and one output port. One input port is used for a microwave or RF input signal having a frequency f1 while the other input port is for a local oscillator signal having a frequency f2. When such signals are applied to the input ports, the following signals are generated at the output port: the original signals f1, f2; the sum and difference of the signals f1 and f2; harmonics of the original signals; as well as the sum and differences of each of the harmonics of the signals f1 and f2. In general, the output signals available at the output of a mixer are provided by equation 1 below:foutput=±M*f1±N*f2,  (1)where M and N are integers and the sum |M|+|N|=“order” of the mixer output signal frequency.
When the mixer is used as an upconverter, the desired output frequency of the mixer may be f1+f2 or f2−f1, for example. Similarly, when the mixer is used as a downconverter, the desired output of the mixer may be f1−f2 or f2−f1. The balance of the signals generated by the mixer are spurious output signals or simply spurs. Such spurs are well known and relate to the inherent characteristics of the mixers, for example, as disclosed in “Effects of Offsets on Bipolar Integrated Circuit Mixer Even-Order Distortion Terms”, by Coffing et al., IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, vol. 49, no. 1, January 2001, pages 23-30.
The spurs at the mixer output port can often times be filtered out with simple low pass or band pass filters. Because of this, the power level of many of the spurs decreases the further the spur frequency is away from the desired output frequency. Thus, due to the low power level, many of the spurs are simply ignored.
The Federal Communication Commission requires that the out of band spurious signals be below a given power level within a given bandwidth for transmitters. One typical requirement for satellite downlinks is that the signals must not exceed 65 dBc within any 4 KHz frequency bandwidth window. Unfortunately, in certain applications this requirement is difficult if not impossible to meet.
FIG. 1 illustrates a typical upconversion example used for a transmitter 20. The transmitter 20 includes two mixer stages, generally identified with the reference numerals 22 and 24. Each mixer stage 22 and 24 includes two input ports and one output port. In this example, a 750 MHz RF input signal, identified with the reference numeral 26, is applied to an input port of the first mixer stage 22. The 750 MHz input signal 26 is to be upconverted to 18.25 GHz. As such, the local oscillator signals for the two mixer stages 22 and 24 are selected as 3 GHz and 16 GHz, respectively.
One output signal of the first mixer stage is 2.25 GHz plus a number of spurious output signal or spurs as discussed above. Some of the spurs are filtered out by a simple intermediate frequency (IF) filter, such as the IF filter 28. The output of the IF filter 28 is applied to the input port of the second mixer stage 24. The output of second mixer stage 24 is typically applied to a power amplifier and subsequently to a transmit antenna (not shown).
Unfortunately, each of the mixer stages 22, 24 produces spurs with significant power levels that can appear in the transmitted output. More particularly, the spurious output signals or spurs from both the first and second mixer stages 22 and 24 are generally significant because of a relatively high power local oscillator signal is required to achieve RF frequency conversion. Typically, the power requirement for the local oscillator signal is at least 20 dB higher than the incoming RF signals. Unfortunately, the relatively high power level spurs produced from the first mixer stage 22 are mixed with the relatively high power level of the local oscillator signals at the second mixer stage 24. Consequently, in addition to the 18.25 GHz desired output signal, relatively high power level spurs are also generated which exceed the 65 dBc power level requirement. Referring to FIG. 2, a relatively high power level spur is generated, identified with the reference numeral 32. This spur 32 represents the leakage from the second mixer stage 24. As shown in FIG. 2, the spur 32 exceeds the 65 dBc requirement. The 65 dBc reference level is identified with the segments 38 and 40. Thus, in order to meet the FCC requirement, a narrow band output filter is required to filter out the spur 32. Such an output filter would be relatively expensive and possibly degrade the transmitter in band performance. As such, there is a need for a mixer configuration, which would reduce the power level of the spur signal resulting from leakage of the local oscillator in the second mixer stage of a transmitter to within acceptable limits.